1. Field of the Invention
The invention relates to a wireless communications network and more particularly to improve the capacity of a reverse link of a communication channel.
2. Background of the Invention
In a wireless network the communication is described in terms of Forward Link (FL) and Reverse Link (RL). The Forward Link and Reverse Links are also referred to as Down Link (DL) and Up Link (UL) respectively. The forward link refers to the transmission from the Base Station (BS) also referred to as Cell to a Mobile Station (MS) and the reverse link refers to the transmission from the MS to a BS. A Mobile Station could be a voice terminal or a data terminal or a combination thereof. Both forward and reverse links transfer or carry content of voice, data, video or any other digital information from one end to the other end. The information transmitted from a Base Station to a specific Mobile Station is uniquely identified in nature compared to the information transmitted to the rest of the mobile stations present in a system at any given time. In code division multiple access (CDMA) systems, the uniqueness is governed by the Orthogonal Walsh functions used to spread or modulate a specific user's data in the Forward link. However, in the Reverse Link, the uniqueness of a specific user's data is governed by the user specific code that will identify the reverse link transmissions.
FIG. 1 depicts a cluster of base stations also referred as cells of a wireless network 1. This cluster consists a center Cell 1 surrounded by plurality of other cells. The identification of a center cell is just a notation to discuss the operation of that cell with respect to its surrounding cells. For example, when the discussion is focused on Cell 1 then the surrounding cells referred to as Tier 1 cells are the cells that are adjacent to all of its sides. In the FIG. 1, these are Cell 2, Cell 3, Cell 4, Cell 5, Cell 6, and Cell 7.
The objective of a system design of the Reverse Link of a CDMA-based system consisting of both voice and data users is to optimize capacity of a system in terms of the data throughput received at the Base Station while maintaining all users at their required quality of service level. The design goal is to make sure that the signal strength received at a base station from every mobile station in that cell is ideally equal to each other. The signal received at the base station from a particular mobile station is interfered with, and its quality of reception at the intended base station is degraded by, the cumulative sum of all the signals received from all the users of that base station referred to as Intra-cell and users of all its surrounding base stations referred to as Inter-cell interference. The design goal of a CDMA system is to minimize all kinds of interference at a given base station.
In the reverse link of a CDMA wireless system, the received signal power at the base station is the main resource that is shared. This resource is bound by the amount of power that can be accumulated due to signal reception from all the mobiles in the network, at the same time maintaining the ability to decode each of the signals. The higher the received power in a signal, the better the decoding ability of that signal at the base station but on the other hand, if the received power from one user is high, it leads to a higher interference to another user who is also transmitting at the same time. The goal of optimizing the level of signal received from each mobile is to balance between these contradictory requirements of expecting to receive a higher power of signal from a user at the same time making sure that the power received from all the other mobiles is decodable. In other words, the goal is maximizing the received power from a particular user while limiting the interference seen by the other users.